Kobe sub, wellbore tubing string apparatus and method

ABSTRACT

To address concerns of kobe plug cap portions being inadvertently removed, for example, by abutment with strings or tools passing thereby, a shielding structure may be employed to protect the cap against inadvertent removal. The port opening tool useful with the kobe plug is selected to overcome the shielding structure to open the kobe plug. To address concerns of a portion of the kobe plug becoming loose in the tubing string inner bore, a capturing mechanism may be provided that captures the cap portion after it is removed to open the kobe plug and prevented the cap portion from becoming loose in the inner bore of the tubing string.

FIELD

The invention is directed to a wellbore apparatus and method and, inparticular a kobe sub, wellbore tubing string and method.

BACKGROUND

In wellbore operations, tubing strings are used having walls with one ormore ports extending therethrough. The ports permit fluid access fromthe tubing string inner diameter and the tubing string's outer surface,which is open to the wellbore.

A kobe plug, also called a break-off plug or a kobe, is closure that canbe mounted at its base over a port with a cap portion extending from thebase. A channel extends through the base into the cap, but is closed offat the cap. The cap portion protrudes from the port and is removablefrom the base to open the port to fluid flow through the channel. A kobeplug is installed in a tubular housing, called a kobe sub, that can beinstalled into a wellbore tubing string. The cap portion of the kobeplug often protrudes into the inner bore of the tubing string.

Generally, the kobe plug is removed by running a tool through inner boreof the string to break off the cap portion. The tool may be a drop bar,a cutter tool, etc. In some embodiments, there they may be concern of acap being inadvertently removed by abutment by a treatment string or atool, as it is passed thereby.

In some other embodiments, the cap portion, when removed from its port,is loose and can interfere with string operations. Such loose portionsof the cap can, for example, jam in tools or in string structures or canobstruct ports.

SUMMARY

In accordance with a broad aspect of the present invention, there isprovided a kobe sub comprising: a tubular body connectable into awellbore tubing string, the tubular body including a wall including anouter surface and an inner surface defining an inner bore and a portthrough the wall; a kobe plug installed in the port with a cap portionaccessible in the inner bore, a base mounted in the port and connectedto the cap portion, a channel extending through the base and closed bythe cap portion; and a shielding structure in the inner bore to shieldthe cap portion from protruding into the inner bore.

In accordance with another broad aspect of the present invention, thereis a provided a method for forming a fluid channel through a tubingstring wall, the method comprising: installing a tubing string in awellbore, the tubing string including a tubular wall including an outersurface and an inner surface defining an open inner bore and a portthrough the wall; a kobe plug installed in the port with a cap portionaccessible in the inner bore, a base mounted in the port and connectedto the cap portion, a channel extending through the base and closed bythe cap portion; and a shielding structure in the inner bore to shieldthe cap portion from protruding into the inner bore; introducing a portopening tool into the inner bore; manipulating the tool to overcome theshielding structure; and removing the cap portion to open the channeland form the fluid channel though the tubing string wall.

In accordance with another broad aspect of the present invention, thereis a provided a tubing string system for installation in a wellborecomprising: a tubular body including a tubular wall with an outersurface and an inner surface defining an inner bore and a port throughthe tubular wall; a kobe plug installed in the port with a cap portionaccessible in the inner bore, a base mounted in the port and connectedto the cap portion, a channel extending through the base and closed bythe cap portion; a shielding structure in the inner bore to shield thecap portion from protruding into the inner bore; and a port opening toolfor overcoming the shielding structure and removing the cap portion fromthe base.

In accordance with a broad aspect of the present invention, there isprovided a kobe sub comprising: a tubular body connectable into awellbore tubing string, the tubular body including a wall including anouter surface and an inner surface defining an inner bore and a portthrough the wall; a kobe plug installed in the port including a basemounted in the port, a cap portion accessible in the inner bore andconnected to the base and a channel extending through the base andclosed by the cap portion, the cap portion being removable from the baseto open the channel to allow fluid passage though the channel from theinner bore to the outer surface; and a capturing mechanism to capturethe cap portion from becoming loose in the inner bore after the capportion is removed from a sealing position on the base.

In accordance with another broad aspect of the present invention, thereis a provided a method for forming a fluid channel through a tubingstring wall, the method comprising: installing a tubing string in awellbore, the tubing string including a tubular wall including an outersurface and an inner surface defining an open inner bore and a portthrough the wall; and a kobe plug installed in the port including a basemounted in the port, a cap portion accessible in the inner bore andconnected to the base and a channel extending through the base andclosed by the cap portion, the cap portion being removable from the baseto open the channel to allow fluid passage though the channel from theinner bore to the outer surface; introducing a port opening tool intothe inner bore; manipulating the tool to remove the cap portion from asealing position on the base to open the channel; and capturing the capportion to prevent the cap portion from becoming loose in the innerbore.

In accordance with another broad aspect of the present invention, thereis a provided a tubing string system for installation in a wellborecomprising: a tubular body including a tubular wall with an outersurface and an inner surface defining an open inner bore and a portthrough the wall; a kobe plug installed in the port including a basemounted in the port, a cap portion accessible in the inner bore andconnected to the base and a channel extending through the base andclosed by the cap portion, the cap portion being removable from the baseto open the channel to allow fluid passage though the channel from theinner bore to the outer surface; a port opening tool for removing thecap portion from a sealing position on the base; and a capturingmechanism to capture the cap portion from becoming loose in the innerbore after the cap portion is removed from the sealing position on thebase.

It is to be understood that other aspects of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various embodiments of the invention areshown and described by way of illustration. As will be realized, theinvention is capable for other and different embodiments and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present invention.Accordingly the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A further, detailed, description of the invention, briefly describedabove, will follow by reference to the following drawings of specificembodiments of the invention. These drawings depict only typicalembodiments of the invention and are therefore not to be consideredlimiting of its scope. In the drawings:

FIGS. 1A, 1B and 1D are sequential schematic sectional views through awellbore with a liner installed therein with port closures being openedto effect a wellbore treatment. FIG. 1C is a sectional view though theinstallation along line II-II of FIG. 1B.

FIGS. 2A to 2G are sequential schematic sectional drawings through aport showing the opening of a port closure using a treatment string.FIG. 2A is a first sectional view through the port and FIGS. 2B to 2Gare sequential views along line I-I of FIG. 2A.

FIGS. 3A to 3C are sequential schematic sectional drawings along a portshowing the opening of a port closure using a treatment string.

FIGS. 4A and 4B are schematic sectional drawings through a port with aport closure installed thereon. FIG. 4A is a first sectional viewthrough the port and FIG. 4B is a section along line II-II of FIG. 4A.

FIGS. 5A to 5D are sequential schematic sectional drawings along a portof a kobe sub showing the opening of a port closure using a port openingtool on a treatment string.

FIGS. 6A to 6E are sequential schematic sectional drawings along a portshowing the opening of a port closure using a pressure conveyed tool.

FIGS. 7A to 7G are sequential schematic sectional drawings along atubing string showing the opening of a plurality of port closures usinga pressure conveyed tool.

FIGS. 8A to 8G are sequential schematic sectional drawings along a portshowing the opening of a port closure using a pressure conveyed plug.

DESCRIPTION OF VARIOUS EMBODIMENTS

The description that follows and the embodiments described therein areprovided by way of illustration of an example, or examples, ofparticular embodiments of the principles of various aspects of thepresent invention. These examples are provided for the purposes ofexplanation, and not of limitation, of those principles and of theinvention in its various aspects. In the description, similar parts aremarked throughout the specification and the drawings with the samerespective reference numerals. The drawings are not necessarily to scaleand in some instances proportions may have been exaggerated in ordermore clearly to depict certain features

FIGS. 1A and 1B illustrate a possible wellbore assembly and method thatemploy kobe plugs 46 as port closures in a tubing string 10.

In wellbore operations, for example, tubing string 10 may be installedin a wellbore 12. The tubing string may have a tubular form and includean upper end 15 a, a lower end 15 b and at least one fluid outlet port16 a extending through the tubing string wall to provide fluidcommunication between the tubing string's inner bore 18 and the tubingstring's outer surface 20, which is open to an annulus in communicationwith the wellbore wall.

In some embodiments, there may be a plurality of fluid outlet portsalong the tubing string, which may include, for example, outlet port 16a, a second fluid outlet port 16 b axially spaced (i.e. downhole oruphole from) the first fluid outlet port and possibly further fluidoutlet ports 16 c.

The ports may be closed to control inner bore fluid conditions and maybe selectively openable, when desired, to permit fluid access betweenthe inner bore and the outer surface. In one embodiment, for example,the ports are each closed by kobe plugs 46 a, 46 b, 46 c (collectivelyreferenced as kobe plugs 46).

A kobe plug is a cap including a cap portion, a base attached to the capportion, and a channel that extends through the base and into the capportion. The kobe plug can be mounted at its base in a port with the capportion protruding beyond the surrounding wall surface. The base can besealed to the port walls, such that the channel creates the flow paththrough the port. Cap portion, however, while in place on the base sealsthe channel against fluid flow. Cap portion, therefore, must be removedto open the port. The cap portion can be removed by shearing, breakingoff, breaking open, pushing through the wall, etc. Sometimes the kobeplug includes a weakened area between the base and cap portion thatfacilitates separation of the cap portion from the base. A kobe plug maybe installed in a tubular body with the cap portion protruding above thesurrounding material of the tubular body. Generally, the kobe plug capportion is accessible in the inner bore of the tubular body. The tubularbody may be connected into a wellbore string. The tubular body with thekobe plug is known as a kobe sub.

A port-opening tool 40, which may take various forms, may be selected topass through the tubing string inner diameter (FIG. 1A) and remove thecap of one or more the plugs 46 to open the ports (FIG. 1B). The toolmay be formed to directly remove (i.e. cut off or abut against, etc.)the cap portions of the plugs 46, as shown, to open fluid access to thechannel through the port protected by the cap. Alternately, aport-opening tool may be formed to drive another structure to cut off orabut against, etc. the cap portions. Tool 40 may be connected to a workstring, as shown, or may be free from any connections to surface. In theillustrated embodiment, a treatment assembly 14 includes a port-openingtool 40 carried on a work string 30. Tool 40 includes a pair ofdiametrically opposed fingers 42 with cutters formed at the outboardtips thereof. Tool 40 can be actuated between an inactive position,where fingers 42 are collapsed and an active position, where fingers 42,are expanded.

In some embodiments, there they may be concern of a kobe plug capportion being inadvertently removed, for example, by abutment with atreatment string or a tool head, as it is passed thereby. In such anembodiment, a shielding structure may be employed to protect the capagainst inadvertent removal and the port opening tool is selected toovercome the shielding structure to open the kobe plug. The shieldingstructure may include a shielding wall such as may be provided anextension of the tubing string wall or by a recess in which the kobe capportion may be recessed. In such systems, the cap portion is recessedbehind the wall to protect it from abutment of tools and strings passingthereby and the port opening tool is selected to overcome the shieldingof the wall by reaching past the wall to access the kobe plug cap.

FIG. 1 illustrate an assembly and a method employing a kobe plug that isprotected by a shielding structure formed as a shielding wall positionedalongside the kobe plug. The shielding wall may, for example, be thewall leading to a recess in the wall of the tubing string or a wallextending out from the tubing string inner wall. In FIG. 1, while kobeplugs 46 a, 46 b, 46 c are opened by shearing the cap portions from thebases, the kobe plugs are protected from inadvertent opening byplacement in a slot 48 along the liner inner wall. The slot may beformed as an elongate recess in the wall of the liner and may be exposedin the inner bore 18. The width of the slot is defined by slot walls 49that extend from the liner inner wall surface to the bottom of the slot.Ports 16 with kobe plugs 46 thereover may be positioned in the depth ofthe slot such that the slot walls protect the kobe plugs from beingengaged by structures, such as assembly 14, moving therepast in theliner inner bore. The depth of the slot and the height of the capportions of kobe plugs may be correspondingly sized such that the capportions substantially do not protrude into the main drift diameter ofinner bore 18. In other words, the height of the cap portion may beselected to be less than the depth of the slot such that the cap portiondoes not protrude inwardly beyond the surfaces of the tubing's innerwall surfaces into which the slot extends.

To further protect the cap portions from accidental opening byinadvertent contact with tools passing thereby, the width of the slotmay be selected such that fingers 42 of tool 14 can enter the slot, butother parts of tool 40 and string 30 cannot. As noted, port-opening tool40 includes fingers 42 with cutters formed at the outboard tips thereof.The fingers and cutters are sized to penetrate between the slot wallsand ride along slot 48 removing the cap portions from the ports byshearing them off (see for example cap 46 c, FIG. 1B).

In this illustrated embodiment, two slots are shown each with aplurality of ports positioned therein. The slots extend along the linerwall between the ports in at least a series of ports 16 a, 16 b, 16 c,such that when the fingers are expanded and each located in a slot, thetool can be moved, arrow P, along the liner with the fingers remainingin their slots to open a plurality of closures without needing torelocate the tool fingers for each port. Of course, the slot may spanfewer ports than those to be opened in one stage of the operation. Forexample, the slot may accommodate only one port. This may require thateach finger run through a number of slots during one stage of theopening operation for a series of ports.

To facilitate the location of a finger 42 in a slot 48, a mule shoerecess 49 may be employed. The mule shoe recess is a groove formed inthe tubing string inner wall. The mule shoe recess has edges that due tothe diameter change from the drift diameter to the larger, groovediameter form shoulders. The shoulder at one end of the groove extendsto form a tapering extension extended along the long axis of the tubingstring. As such, the groove has generally a tear drop shape, with an endtapering from a larger width to a narrower width. The groove, therefore,can act as a funnel-like guide for tool positioning. A key on tool 40may be landed in the mule shoe recess and may be guided to the correctrotational orientation as guided into the tapering extension, by movingthe key along the shoulder, to locate fingers 42 in their slots.Alternately, the mule shoe recess 49 may be positioned with the taperingextension leading directly into slots 48 such that a finger may becorrectly positioned in a slot by being moved to follow the groove'sshoulder.

In FIG. 1, a series of ports in the liner string are all opened before awellbore treatment is undertaken. Any number of ports can be opened,such as one to four or more, and then a wellbore fluid treatmentoperation, such as a fracing operation is initiated. Because the kobeplug closures for the ports are recessed and, therefore, shielded fromaccidental opening, there is little risk of ports other than thoseintended being opened. The wellbore fluid treatment operation can beinitiated down the annular area or through the tubing string tosimultaneously treat the wellbore using the ports in the series. Theports may be opened to one or more intervals in the well. The system mayuse a limited entry type technique to ensure the frac fluid isappropriately distributed between the opened ports. In a limited entrysystem, a sized nozzle is installed in at least some of the ports in theseries to allow distribution of the fluid in an appropriate and plannedmanner through all the ports in the series that are to be opened andfraced together.

To facilitate understanding of how the string may be employed adescription of one possible method follows. The method includes runninginto the well with liner 15 including at least one series of selectivelyopenable ports 16 a, 16 b, 16 c. The liner can be set in the well tocreate an annulus 13 between the wellbore wall 12 and the liner. Ifdesired, without cementing the annulus, isolated intervals can beestablished along the well by setting liner-conveyed packers 26′, 26″,26′″ to create annular seals in the annulus. The space between eachadjacent pair of packers represents an isolated interval and the portsare each positioned to provide communication from the liner inner bore18 to an isolated interval. Some isolated intervals, such as thatbetween seal 26′ and seal 26″, can be accessed by more than one axiallyspaced port. The series of ports can be in the same interval, with apacker on either side of the series, but not separating annularcommunication between the ports of the series, or, as shown, packers canbe installed to separate one or more of the ports in the series from oneor more other ports in the series.

In this illustrated embodiment, ports 16 a, 16 b, 16 c when run in areeach closed by kobe plug closures 46 a, 46 b, 46 c, but can beselectively opened, as described above, by operation of port-openingtool 40 carried on treatment string assembly 14 that can be movedthrough the liner inner bore. In this embodiment, tubing string 30 hasan inner conduit in fluid communication with surface and a closed bottomend 30 a and assembly 14 also carries a seal, such as asettable/releasable packer 32, carried on the string and actuable tocreate a seal between the tubing string and liner 15, a port 38providing fluid communication, when opened, between the outer surface ofthe tubing string and the inner conduit above the seal (on a side of theseal opposite bottom end 30 a). Of course, this being only an example ofthe wellbore assembly using a kobe plug, the treatment string componentscan be selected according to various options.

When it is time to begin a wellbore fluid treatment, such as a fracingoperation, the treatment string can be moved or introduced open andtreat though the ports along the string. Generally, the ports at thedistal end of the well are employed first and the fracing operation isconducted moving up through the well.

Eventually preparations are carried out for fluid treatment throughseries of ports 16 a, 16 b, 16 c, First, the series of ports are opened(FIGS. 1B and 1C) to provide for fluid communication between inner bore18 and annulus 13. To do this, the port-opening tool 40 can be moved tothe ports to open their closures 46. For example, port-opening tool 40can be moved, arrow P, from port to port in the series of ports and canactuate the ports to open. In one embodiment, as the shifting tool ismoved through the liner inner bore, the port-opening tool, if notalready in position, can be activated into an active position to expandfingers 42. Activation of tool 40 can be by pressure, by flow, byshearing or by directional movement up or down. Fingers 42 are thenlocated in slots 48 (FIGS. 1B and 1C) and the cap portions of kobe plugs46 are removed by pulling, arrow P, fingers 42 through the slots 48 toshear off the caps. After the series of ports 16 a, 16 b, 16 c are open,the string 30 is moved downhole below the lowermost of the ports in theseries, in this case port 16 c, and the seal member 32 is then set toseal off the annular area 36 between the liner and the string to isolateall the zones below from the series of opened ports (FIG. 1D).

Once all the selected ports are opened and the liner below the openedports is sealed, then fluid can be introduced, arrows F, to treat thewellbore through the opened ports. For example, as shown, one or morewellbore intervals can be fraced simultaneously through the openedseries of ports. Ports 16 a, 16 b, 16 c in the series can include valves60 therein to provide for limited entry and, thereby, appropriatedistribution of fluids through the ports in the series. Wellboretreatment fluids can be introduced from surface through the annular area36, as shown, and/or through the tubing string inner bore, exitingthrough port 38. In the illustrated embodiment, wellbore fracing fluidsare introduced from surface through the annular area 36 and port 38 isopen to monitor downhole pressure conditions. String 30 remainspressurized to ensure fluids do not circulate upwardly therethrough.Fracing fluids F exit through ports 16 a, 16 b, 16 c into the annulus 13and into contact with the open hole wellbore wall along the intervalsbetween packers 26′ and 26″ and between packers 26″ and 26′″.

The foregoing process can be repeated at a plurality of series of portsmoving up through the liner. For example, after fluid treatment, thepacker 32 can be unset and the treatment string assembly may be movedupwardly in the liner to a next series of one or more ports, theport-opening tool can be manipulated to shear off the caps in that nextseries, the treatment string assembly can be moved below the lowermostof the opened ports in that next series where the sealing member can beset to seal the annular area and a fluid treatment can be conductedthrough the opened ports.

The process and system therefore allows an operator to access and treatmultiple intervals at the same time and, so, provides significantsavings in terms of time and cost, without a significant risk of kobesbeing inadvertently removed by assembly 14, including inactive tool 40or string 30.

While FIG. 1 illustrate a wellbore assembly with kobe plugs that areshielded against inadvertent opening, in addition or alternately, therethey may be a concern of a portion of the kobe plug being loose in thetubing string inner bore since that portion, for example cap portion 46c′, may interfere with (i.e. jam, obstruct, etc.) tubing stringoperations. As such, in an embodiment, the kobe plug may be configuredwith a capturing mechanism such that a part of the kobe plug, such asthe cap portion or a portion thereof, that is removed to open a portremains captured and is prevented from becoming loose in the liner innerbore.

In one embodiment, the part that is removed to open the kobe plug may becaptured by being forced outwardly away from the inner bore toward theouter surface of the string.

Alternately or in addition, the part that is removed to open the kobeplug may be captured by being stored after it is removed. The part maybe stored by the port-opening tool, by remaining attached to theremainder of the kobe plug, by storage in the tubing string, etc.Storage options may include hinge connections, snap-type retainers,frictional retaining mechanisms, magnetic attraction, etc.

Examples of captured kobe plugs follow, including some that also areshielded against inadvertent opening.

For example, a captured and shielded kobe plug is shown in FIGS. 2A to2G. In this embodiment, a port 116 in a tubing string wall 115 has aclosure in the form of a kobe plug 146 including a cap portion 146 a anda base 146 b. The kobe plug includes a channel 146 c through the basethat extends up into, but is sealed off by, cap portion 146 a. Kobe plug146 is installed in port 116 with seals 146 d between the port walls andbase 146 b. Channel 146 c actually forms the flow path area of the port,but is normally closed by the cap portion, which overlies and sealsaccess to the channel.

In this embodiment, kobe plug 146 is shielded against inadvertentopening. In particular, a slot 148, defined between slot walls 149, maybe formed in the tubing string wall 115 exposed in the tubing string'sinner bore. The width of the slot, which is the distance between theslot walls 149, can be selected with consideration as to the size of thetreatment string components such that only selected components can passinto the valley. For example, the slot width (from wall 149 to wall 149)can be less than the diameter of the tubing string such that the slot issized to prevent a coiled tubing string, such as string 30 of assembly14 in FIG. 1A, from entering the valley. Port 116 and kobe plug 146 maybe positioned in the depth of the slot such that the slot walls protectthe cap from being engaged by structures moving therepast in the tubingstring inner bore. In such an embodiment, a finger 142 can be carried onthe port-opening tool that can reach into the slot and open the port byopening cap portion 146 a to expose channel 146 c. The illustrated kobeplug is opened by removing a portion of the cap portion. While thatportion could be removed completely from the port and released into theliner, in this embodiment it is desirable to limit the release of debrisinto the tubing string as such debris can interfere with tubingoperations. As such, the portion removed to open the kobe plug iscaptured.

For example, as shown, finger 142 operates to open the kobe plug bybreaking cap portion 146 a open and pushing the cap portion out into theport. In particular, finger 142 can be inserted into the slot betweenwalls 149 (FIG. 2B) and moved, as by pulling or pushing, past cap 146.In so doing, finger 142, as it passes, can bear against and break openthe cap to create a flap 146 a′ that is pushed out into the port (FIGS.2C to 2E). After acting on the cap portion, finger 142 may moved toallow fluid access to port 116. For example, as shown in FIG. 2F, thefinger may be moved with the tool to another position in the well orremoved from the well. The flap 146 a′ may be removed completely fromits position over the port or, as shown in FIG. 2E, may remain stored,as by being connect at a hinge 150 to the remainder of the kobe plug.However, regardless, the integrity of the cap is compromised such thatchannel 146 c is opened therethrough and fluid, such as fracing fluid F,may be pumped out through the opened cap and its associated port 116. Asthe fluid passes out through the port, the flap may be pushed out of theway and may break free at hinge 150 such that the flap is removedaltogether (FIG. 2G). However, the force of the fluid pushes the flapthrough port 116 such that it is expelled from the tubing string.

Thus, in the embodiment of FIG. 2, the portion of the kobe plug that isremoved is captured. Initially, it is captured by remaining connected athinge 150 and thereafter it is captured in the annulus away from thetubing string inner bore. In both conditions, the removed part 146 a′ ofthe cap portion cannot interfere with the operations in the inner boreof the tubing string.

Finger 142 may be sized to fit into slot 148 and move therealong to acton kobe plug 146. Finger 142 may have a radiused or chamfered leadingend 142 a such that it tends not to get caught up on discontinuities inthe tubing string or slot. Alternately or in addition, cap 146 can beformed to present a ramped surface such that finger 142 tends to moveover the cap rather than being caught up on it. Also, this forming ofthe finger and/or the cap tends to urge the cap outwardly through theport rather causing the cap to move into the inner diameter of thetubing string. For example, this forming causes leading end 142 a of thefinger to ride up onto the cap portion, which tends to push the openedflap 146 a′ out through channel 146 c.

Finger 142 may always protrude in an active position from theport-opening tool or may be moveable from a retracted position to anactive position. In one embodiment, for example, the tool may include afinger and a shifting tool to move the finger between a retractedposition and an exposed, active position. The shifting tool may, forexample, be a 360° collet shifting tool that activates the finger. Thefinger can be moved into an active position by the shifting tool, movedinto the valley and moved across the cap to remove the cap.

Another kobe plug that operates to direct the opened cap portion awayfrom inner bore is shown in FIG. 3. In this embodiment, a kobe plugclosure includes a plug installed in a channel that may be moved out ofa sealing position to open the port. For example, a ball-bearing plug156 may be installed, as by press fitting, in a narrowed portion of atapered port 158. Port 158 may be formed, as shown, by an insert sealedin a hole through the tubing string wall 115 or by forming the holeitself. The plug is installed to have a contact portion 156 a protrudingat least a distance into the ID of the tubing string such that a toolpassing through the tubing string inner bore 118 may contact the plug.The installation of plug 156 in port 158 can be selected to hold theinternal pressures intended to be used in the tubing string. However,plug 156 can be removed from port 158, to open the port, by applying amechanical force, greater than that force exerted by any operationalfluid pressure, against contact portion 156 a to push it out. Port 158tapers inwardly from the tubing string outer surface to the inner boresuch that the plug can more easily pass outwardly from the port once itis freed from its installed position. In such an embodiment, theport-opening tool can include a structure such as an anvil 160 that canbe moved over the plug to apply a pressure against its exposed portion156 a to drive the plug radially outwardly. The pressure frees the plugfrom its installed position in port 158. After the anvil passes, even ifthe plug is not fully removed from the port it is loosened and fluidpressure, for example fracing fluid F, can fully eject the plug from theport (FIG. 3C). The embodiment, of FIG. 3 is included to illustrateanother embodiment of a kobe sub having an opened part that is capturedby directing it outwardly toward outer surface 115 a away from innerbore 118 of the tubing string. Of course, while not illustrated, plug156 and port 158 may be installed in a recessed area to protect the plugfrom inadvertent strikes by tools passing thereby. However, the lowprofile presented by the plug's contact portion 156 a may not readily beaffected by occasional abutment of tools passing thereby.

Another kobe plug closure is illustrated in FIGS. 4A and 4B, which isboth shielded and captured. As shown, a kobe plug may be shielded byshielding wall such as for example, a recessed positioning in a slotwith walls extending up alongside the kobe plug cap and/or by theprovision of an extension of the wall inner surface surrounding the kobeplug that partially overlies the cap portion.

In the illustrated embodiment of FIG. 4, kobe plug 186 includes ashearable cap portion 186 a that after shearing becomes captured in aholding area in the tubing string wall 115. In one embodiment, forexample, kobe plug is 186 installed in a slot 188 in inner wall surface115 b such that cap portion 146 a is recessed in the slot. In thisembodiment, kobe plug can only be opened by inserting a port-openingtool structure (not shown) into the slot to apply a force, arrow S, toshear off the cap portion at the weakened shear plane 186 e. In thisembodiment, the slot's side walls include returns 189 that partiallyoverlap cap portion 186 a. While a port opening tool can be selected topass through returns into the slot to contact the kobe plug cap 186 a,returns 189 act as keepers, forming an opening to slot 188 that issmaller than the dimensions of cap portion 186 a. Thus, even after beingsheared off from the remainder of kobe plug, the cap portion is capturedand cannot pass out of the slot. The sheared cap portion may simply bepushed aside in slot 188 such that the channel 186 c of the kobe plugbecomes opened. Alternately, slot 188 may be formed store the shearedcap in a fixed position away from the base. For example, slot 188 may beformed such that the sheared cap portion becomes frictionally jammed ina restricted portion 190, such as a narrowed or high friction (i.e.roughened, deformable, etc.) portion of the slot or a blind end of theslot. As another alternative, slot 188 may be formed to open into acavity 191 in wall 115 in which the sheared cap portion can be retained.The slot may be formed to direct the sheared cap into the cavity or thecap may be formed to urge itself into the cavity. For example, the slotand/or the cap may include a deflection structure, as desired, to directthe sheared cap into the retaining cavity. Alternately, or in addition,the slot and/or the cap may be formed such that the cap can more readilymove into the cavity than out of the cavity such that the cap isretained in the cavity. Capturing nubs, keys, restrictions, deflections,slots, etc. can be employed as desired in the slot and/or on the cap forthis purpose.

Still other recessed and captured kobe plug closures can be employed,such as that shown in FIG. 5. In such a system, the cap portion 346 acan be protected from abutment of tools and strings passing thereby andis removable from its port to open it and the sheared cap remainscaptured such that it is not released into the tubing string. Forexample, as shown, a port 316 can have a closure in the form of a kobeplug. The kobe plug includes a base portion 346 b mounted in the portand a cap portion 346 a that can be sheared from the mounted, baseportion. An inner channel 346 c extends up through the base portion andinto cap portion 346 a. While the channel opens on the end of the baseportion at outer wall surface 315 a, the channel is closed at its otherend by cap portion 346 a. The cap portion controls the ability of fluidto flow through the inner channel forming the port. In particular, whencap portion 346 a is in place, connected to base portion 346 b, fluidcannot flow through the port, it being prevented by the solid form ofthe cap and seals 346 d encircling the base portion. However, when capportion 346 a is sheared from the base 346 b, the channel is exposed andfluid can flow there through. While alternatives are possible, in oneembodiment, the cap portions 346 a, 346 b may be formed as a unitarypart and have a solid, fluid impermeable, but weakened area 346 ebetween them.

A sleeve 380 is positioned over port 316 and cap 346. The sleeveincludes an inner surface exposed in the inner diameter 318 of thetubing string 315 and an outer surface, facing the tubing string innerwall and including a surface indentation 380 a. Indentation 380 a issized to accommodate cap portion 346 a of the sleeve therein and isformed such that cap portion 346 a remains at all times captured by thesleeve (i.e. cannot pass out from under the sleeve). Sleeve 380 ismoveable within the tubing string inner bore from a position overlyingthe port and accommodating cap portion 346 a, in indentation 380 a. Onits inner facing, exposed surface, the sleeve can be contacted by asleeve shifting tool, a portion of which is indicated at 342. Forexample, sleeve 380 may include a shoulder 380 b against which tool 342can be located and apply force to move the sleeve. Sleeve 380 may belocated in an annular recess 381 in order to ensure drift diameter inthe tubing string. This positioning also protects the sleeve frominadvertent contact with tools during movement of such tools past thesleeve. Sleeve 380 can include a lock to ensure positional maintenancein the string. For example, sleeve 380 may carry a snap ring 382positioned to land in a gland 388 in the tubing string inner wall, whenthe snap ring is aligned with the gland.

Sleeve 380 can be moved to shear the cap and open the port, whileretaining the sheared cap portion 346 a in the indentation. For example,during run in and before it is desired to open the port to fluid flowtherethrough (FIG. 5A), the cap's cap portion 346 a remains connectedand sealed with base portion 346 b. Sleeve 380 is positioned over theport with portion 346 a positioned in indentation 380 a.

When it is desired to open the port, sleeve 380 can be moved, as bylanding tool 342 against the sleeve, such as against shoulder 380 b ofthe sleeve, (FIG. 5B) and, applying a push, pull or rotational force tothe sleeve to move it along the tubing string (FIG. 5C). When sleeve 380moves, force is applied to the cap cap portion 346 a by abutment of theside walls of the indentation against portion 346 a. Since cap portion346 a is urged to move, while base 346 b is fixed, portion 346 a becomessheared from base portion 346 b. While removal of cap portion 346 aopens the port, the sleeve 380 with the sheared cap portion 346 acaptured therein can be slid until it fully exposes port to the innerbore. For example, sleeve 380 can be moved until it becomes locked, asby snap ring 382 landing in gland 388 in a displaced position, while capportion 346 a remains captured in indentation 380 a. Fluid, such asfracing fluid F, may be pumped out through the channel forming port 316,which is exposed by opening the cap (FIG. 5D).

Another sub of a tubing string 415 is shown in FIG. 6, which includes arecessed and captured kobe cap closure 446. In this embodiment, the portopening tool is an actuation sleeve 440 (similar to a cutter sleeve),which is moveable along the tubing string by fluid pressure to act onthe cap closure. Actuation sleeve 440 need not, therefore, be connectedto surface.

Kobe cap 446 is similar to the kobe cap 346 of FIG. 5. For example, kobecap 446 includes a base portion 446 b mounted in a port 416 and a capportion 446 a that can be sheared from the base portion. An innerchannel 446 c extends up through the base portion and into cap portion446 a and the inner channel, while normally closed is opened by removalof the cap portion.

A sleeve 480, in this case in the form of a c-ring, is positioned overport 416 and cap 446. Forming the sleeve as a c-ring facilitatesinstallation and, as will be described later, can simplify some otheroperations as well. The sleeve includes an inner surface exposed in theinner diameter 418 of tubing string 415 and an outer surface, facing thetubing string inner wall. An indentation 480 a is formed on the outersurface, which is sized to accommodate cap portion 446 a of the sleevetherein and is formed such that cap portion 446 a remains at all timescaptured by the sleeve (i.e. cannot pass out from under the sleeve).Sleeve 480 is moveable within the tubing string inner bore from aposition overlying the port and accommodating in indentation 480 a thecap portion 446 a, while it is still connected to base 446 b. Sleeve 480can be moved to apply a force against cap portion 446 a to open theport, while retaining the sheared cap portion 446 a in the indentation.Thus, when the sleeve is moved, the cap is sheared by the sleeve but thesheared cap is not released into the well and the sheared cap cannotinterfere with intervention operations.

Sleeve 480 is moved by actuation sleeve 440. Sleeve 480 has a shoulderexposed in the inner bore that can be contacted by the actuation sleeve.For example, shoulder 480 b protrudes slightly into the open diameter ofstring 415 and presents a surface against which the leading edge ofactuation sleeve can engage and apply force to move sleeve 480. Sleeve480 may be located in an annular recess 481 to provide some protectionfrom inadvertent contact with tools moving past. Sleeve 480 may also besecured by a shear pin 483 to further protect it against inadvertentmovement.

The assembly can include a lock mechanism ensure positional maintenancein the string. For example, the sub may include a gland 488 adjacent theinstalled location of kobe plug 446 into which sleeve 480 can beretained after it is moved to open the kobe plug. Since sleeve 480 isformed as a c-ring it can be selected to act as a snap ring with anability to expand into gland 488, when the sleeve is aligned with thegland. An end wall 481 a of recess 481 may be formed with an acuteangular face to force the sleeve to expand out into the gland if itdoesn't automatically do so. Once in gland 488, the spring force in thec-ring construction of sleeve 480 prevents the sleeve from slipping backover port 416.

Actuation sleeve 440 is positioned in tubing string 415 axially spacedfrom kobe cap 446 and includes a seat 451 against which a plug, such asa ball 452, as shown, a dart, etc., can be landed and seal. Actuationsleeve 440 can be driven to move by fluid pressure through tubing string415 when plug 452 is landed on seat 451. Once the plug lands, a pressuredifferential is established that pushes the cutter assembly throughinner bore 418 to act on sleeve 480. Actuation sleeve 440 can includeseals 453 about its outer diameter that facilitate its pressure drivenmovement. Since actuation sleeve 440 presents an open bore, itsubstantially doesn't restrict access through it to the tubing stringbelow. Thus, while the actuation sleeve could be introduced when needed,alternately, actuation sleeve 440 can be installed in the tubing stringbefore it is run in and has no effect on operations until ball 452, orother plugging device, is landed in seat 451.

During run in and before it is desired to open port 416 to fluid flowtherethrough (FIG. 6A), the cap's cap portion 446 a remains connectedand sealed with base portion 446 b and sleeve 480 is positioned over theport with portion 446 a positioned in indentation 480 a.

When it is desired to open the port, sleeve 480 can be moved bylaunching ball 452 to land in actuation sleeve 440. Once the ball islanded in the seat of the actuation sleeve, actuation sleeve 440 isdriven by fluid pressure to sleeve 480. When the actuation sleevearrives at sleeve 480, it hits shoulder 480 a (FIG. 6B) and overcomesthe holding force of shear pin 483 to move sleeve 480. This shears capportion 446 a from the base to open channel 446 c. (FIG. 6C) After it issheared, cap portion 446 a remains captured in indentation 480 a andmoves with sleeve 480.

After it is sheared out, sleeve 480 may continue to be moved byactuation sleeve 440 until it is clear of channel 446 c through theport. In this embodiment, sleeve 480 is moved over gland 488 and mayexpand into the gland (FIG. 6C). Actuation sleeve 440 can also bestopped in tubing string 415. Alternately, as shown, it may be intendedthat actuation sleeve 440 continues to move down the tubing string forother purposes downhole or to be stopped by a landing sub. Gland 488 mayhave a depth that permits sleeve 480 to expand out of reach of actuationsleeve 440 such that shoulder 480 b moves out of contact with theactuation sleeve. If sleeve 480 fails to expand into gland 488, it willcontact face 481 a of recess 481 (FIG. 6D) and be forced to expand intogland 488 such that actuation sleeve 440 can move past (FIG. 6E).

As soon as cap portion 446 a is sheared from base 446 c (FIG. 6C), fluidhas access to channel 446 c. When seals 453 of the actuation sleeve aremoved past the channel, fluid can be injected out through the channel.However, in some embodiments, for example, where actuation sleeve 440requires pressure to be maintained, it may be desirable to restrict flowout through channel. As noted above, a limited entry nozzle 493 may beinstalled in port 416, such as may be formed of carbide or other hardmaterials and shaped to limit the flow through channel 446 c. Inaddition or alternately, a further fluid flow limiter 495 may beemployed such as a burst disc or an erosion washer. Limiter 495restricts or prevents fluid flow through port 416 until a further forceis applied to overcome the limiter. The force may be a burst pressure oran erosive force. For example, an erosion washer is formed of a materialcapable of being eroded in a fluid flow and includes a small diameteraperture therein. While limited fluid flow is permitted through theaperture, that flow causes erosion of the washer body to eventuallypermit full flow though channel, as limited by nozzle 493. Limiter 495thus provides a delay suitable to maintain fluid pressure driving forcefor the actuation sleeve 440.

After use or whenever it is desired to remove the inner diameterconstriction caused by actuation sleeve 440 in the well, the actuationsleeve may be milled up to provide a full ID access to tubing string415. Thus, for example, the actuation sleeve can be manufactured fromcast iron or polymeric materials which are millable. While millable,cast iron is less millable than some polymers. Thus, this inventionutilizes a combination of materials to ensure proper durability but tofacilitate milling. For example, a composite material can be used forthe non-pressure containing section of the actuation sleeve, thusreducing the milling time for this part, while more durable materialssuch as cast iron are used for the pressure holding sections, like theseat.

Sleeve 480, which can become fully recessed in the tubing string, neednot be milled.

The above-noted sub may be useful in an assembly where a plurality ofports are to be opened in the same operation. For example, withreference to FIG. 7, a string can be prepared with a plurality of kobesubs 409 each including one or more kobe plugs 446. The kobe plugs areeach installed in a port through the kobe sub wall. The assembly mayfurther include an actuation sleeve 440 installed on one side of plugs446 and a catching sub 496 installed on the opposite side of plugs 446.

Kobe plugs 446 may have the form as described above with sleeves 480installed thereover that can be sheared out by actuation sleeve 440 to,thereby, open flow channels through the kobe plugs. Fluid flow limiters495 are also installed to limit fluid flow through the channels, oncethey are opened.

While FIG. 7 show only one series of ports to be opened by an actuationsleeve, it is noted that string 415 may include a number of similarintervals above and/or below that illustrated having one or morerecessed and capture kobe-plugged ports and an actuation tool to openthem.

FIG. 7A shows the tubing string assembly as it is run in the well.Actuation sleeve 440 is pinned in a position axially spaced from subs409. Kobe plugs 446 are intact and, therefore, sealed against fluid flowtherethrough and their cap portions are each protected beneath sleeves480. As it is usual to inject fluid from surface to pass through innerbore 418, when run in and positioned, actuation sleeve 440 is upholefrom subs 409.

When it is desired to open the ports in which Robe plugs are installed,a plug such as ball 452 is launched and lands in the seat of sleeve 440(FIG. 7B). The pressure differential that is generated shears outactuation sleeve 440 and drives it through the string.

When actuation sleeve 440 arrives at the first kobe plug (FIG. 7C), theactuation sleeve hits the shoulder of sleeve 480 and shears the sleevefree of its pinned connection, which shears off the cap portion of thekobe plug to open access to its channel 416 (FIG. 7D). The actuationsleeve continues to push sleeve 480 until it expands into its gland 488.

Actuation sleeve 440 continues on through tubing string 415. It is notedthat once the cap portion of the kobe plug is sheared, channel 416 isopen to fluid flow. However, fluid flow limiters 495 can be employed tomaintain sufficient pressure holding capability in the string. Forexample, limiters 495 here are illustrated as erosion washers thatpermit a small amount of fluid arrow F1 to pass, but any pressure lossby such flow is insufficient to stop the movement of actuation sleeve440. Thus, actuation sleeve 440 continues, as driven by fluid pressure,and shears the remaining sleeves 480 to open the remaining kobe plugs(FIG. 7F). Eventually, as shown in FIG. 7G, actuation sleeve 440 landson the catching sub 496 where it hits a shoulder and is stopped. Sinceactuation sleeve 440 with ball 452 therein creates a seal against fluidpassage, actuation sleeve 440 creates a seal that pressure isolates thetubing string below from that above and diverts fluid pressure to theopened channels 416 of the kobe plugs.

As noted, limiters 495 permit a small amount of fluid (arrows F1) topass once the kobe plugs are opened, but eventually the flow erodes thelimiters such that substantially full flow (arrows F2) is achieved.Limiters 495 provide for delayed opening of the channels to ensure thepressure holding capability of the string is maintained long enough thatthe actuation sleeve can act on all the kobe plugs in the series.

A further variation of a recessed and captured kobe plug is similar tothat of FIG. 7 but doesn't use an intermediate actuation sleeve.Instead, a plug, such as a ball, a dart or the like, is used as the portopening tool to directly actuate the protecting sleeve for the kobeplug. As such, this kobe sub is directly ball actuated.

In such a system, the kobe plug cap is protected from abutment of toolsand strings passing thereby by a sleeve, but once removed to open theport in which it is installed, the cap remains captured such that it isnot released into the tubing string nor into the annulus. In thisembodiment, the sleeve shielding the kobe plug includes a ball seatformed on its inner diameter. A ball can be launched to hit the seat,the ball being selected to have a diameter greater than that of thediameter of the seat. Once the ball hits the seat the diameterdifferential ensures that the ball at least initially cannot pass thoughthe seat and that force is translated to the sleeve under which the kobeplug is protected. The force moves the sleeve, which shears the top ofthe kobe plug to access the annulus of the tubing. This access allowsthe formation about the string to be treated, for example fraced.

A system of these captured lobes can be used to stimulate a largesection along the well since the ball seat and or ball can be formed tobe deformable to allow the ball to act on a number of ball seats as ittravels along the string and the ports can be configured to besubstantially pressure holding, even after opening, as restricted bynozzles, flow limiters such as an erosion washer (i.e. an erodible diskwith a small hole through it), etc. to ensure that sufficient pressurecan be maintained to move the ball and the sleeves. Once the ball hasopened all of the kobe sleeves it lands on, fluid can be diverted to theopened ports. A seal may be established in the string below the openedkobe plugs to pressure isolate the opened ports from ports below and toassist in the diversion of fluid to the opened ports. For example, wherecollapsible ball seats are employed on the kobe protecting sleeves, anon-collapsible ball seat may be installed in the string. In oneembodiment, the non-collapsible ball seat may serve a dual purpose, forexample, it may be the ball seat of a standard sleeve closed port andmay open that port as well once it lands.

Erosion washers and nozzles may be employed together. In such acombination, the erosion washers initially substantially prevent flowthough the ports. As the stimulation progresses, however, the discserode away leaving the port fully open to the diameter of the nozzle.

Such a recessed and captured kobe plug is shown in FIG. 8. As notedabove, in this embodiment, a pressure conveyed plug 542, such as a ballas shown, is used alone as the port opening tool. Thus, each sleeve 580,that shields the kobe plug cap portion 546 a, includes a ball seat 380 bthat catches the ball to move the sleeve.

For example, as shown, a sub including a tubular wall 515 with a port516 through its wall may include an upper end 515 a and a lower end 515b each formed for connection into a tubing string. Port 516 can have aclosure in the form of a kobe plug. The kobe plug includes a baseportion 546 b mounted in the port and a cap portion 546 a that can besheared from the mounted, base portion. An inner channel extends upthrough the base portion and into cap portion 546 a, but is closed bycap portion. The integrity of cap portion controls the ability of fluidto flow through the inner channel forming the port. In particular, whencap portion 546 a is in place, connected to base portion 546 b, fluidcannot flow through the port, that flow being prevented by the solidform of the cap portion and the seals encircling the base portion.However, when cap portion 546 a is sheared from the base 546 b, thechannel is exposed and fluid can flow through the channel, which createsthe flow opening of port 516 between inner bore 518 and outer surface520, which is open to the formation 512.

While alternatives are possible, in one embodiment, the cap portions 546a, 546 b may be formed as a unitary part and have a solid, fluidimpermeable, but weakened area between them.

A sleeve 580 is positioned over port 516 and kobe plug 546. The sleeveincludes an inner surface exposed in the inner diameter 518 of thetubing string 515 and an outer surface, facing the tubing string innerwall and including a surface indentation 580 a. Indentation 580 a issized to accommodate cap portion 546 a of the kobe plug therein and isformed such that cap portion 546 a remains at all times captured by thesleeve (i.e. cannot pass out from under the sleeve). Sleeve 580 ismoveable within the tubing string inner bore from a position overlyingthe port and accommodating cap portion 546 a while it is still connectedto the base portion, in indentation 580 a. On its inner facing, exposedsurface, the sleeve can be contacted by a sleeve shifting tool, such asa fluid conveyed plug 542 (such as a ball, a dart or the like). Forexample, sleeve 580 may include a seat 580 b against which plug 542 canbe landed and can create a substantial seal to establish a pressuredifferential across the sleeve. The pressure differential, onceestablished, applies a force to move the sleeve.

Although not shown, sleeve 580 may be located in an annular recess inorder to enlarge the drift diameter in the tubing string. Thispositioning also protects the sleeve from inadvertent contact with toolsduring movement of such tools past the sleeve.

Sleeve 580 can include a lock to ensure positional maintenance in thestring. For example, sleeve 580 also may pinned, as by a shear pin (notshown), to further act against inadvertent movement out of its initialrun in position. Alternately or in addition, sleeve 580 may have a lockthat engages after the sleeve has been moved to open the kobe plug. Forexample, sleeve 580 may carry a snap ring 582 positioned to land in agland 588 in the tubing string inner wall, when the snap ring is alignedwith the gland.

Sleeve 580 can be moved to shear the cap and open the port, whileretaining the sheared cap portion 546 a in the indentation. For example,during run in and before it is desired to open the port to fluid flowtherethrough (FIG. 8A), the cap's cap portion 546 a remains connectedand sealed with base portion 546 b and sleeve 580 is positioned over theport with portion 546 a positioned in indentation 580 a.

When it is desired to open the port, sleeve 580 can be moved, as bylanding a plug 542 against the sleeve, such as seat 580 b of the sleeve,(FIG. 8B) and, applying a push force to the sleeve to move it along thetubing string (FIG. 8C). When sleeve 580 moves, force is applied to thecap portion 546 a by abutment of the side walls of the indentationagainst portion 546 a. Since cap portion 546 a is urged to move, whilebase 546 b remains fixed, portion 546 a becomes sheared from baseportion 546 b. While removal of cap portion 546 a opens the port andsome amount of fluid can pass under the sleeve, which has no seals, andout through port, the sleeve 580 with the sheared cap portion 546 acaptured therein can be slid until it fully exposes port to the innerbore. For example, sleeve 580 can be moved until it becomes locked, asby snap ring 582 landing in gland 588, in a displaced position, whilecap portion 546 a remains captured in indentation 580 a.

Fluid, such as fracing fluid F, may be pumped out through the channelforming port 516, which is exposed by opening the cap (FIG. 8D).

After the sleeve moves, the plug 542 can pass through the sleeve tocontinue downhole, where it may actuate further sleeves and/or land tocreate a seal. In this embodiment, for example, seat 580 b is formed tobe collapsible such that once it has been employed to move the sleeve;the seat can be overcome by the plug to allow it to pass furtherdownhole. Seat 580 b may for example be formed of protrusions such asdogs that, while initially supported in an active position, may becollapsed radially outwardly, after the sleeve moves. The protrusions,for example, can include a protruding end 580 b′ and a back end 580 b″and can be carried in slots 597 in the sleeve. The protrusions, whileretained in the slots, can slide radially in and out through the slots.When seat 580 b is active, the protrusions protrude inwardly into theinner diameter of the sleeve to define the active diameter of the seat.However, a recess 581 is formed in the tubing body and is positionedrelative to the sleeve and the protrusions such that, the protrusionscan drop into the recess after the sleeve is moved, arrow M, by theplug. When the protrusions drop into the recess, they retract out of ablocking position for the plug and the plug is free to move past theprotrusions (FIG. 8D).

Where a plurality of ports are to be opened by plug 542 along the lengthof the tubing string, the system may use a limited entry type techniqueto ensure the frac fluid is appropriately distributed between the portsand to ensure that sufficient pressure is retained to continue to moveplug 542 through the string. In a limited entry system, a sized nozzle593 is installed in at least some of the ports in the series to allowdistribution of the fluid in an appropriate and planned manner throughall the ports in the series that are to be opened and fracedsimultaneously. In one embodiment, as shown, another limiting system maybe employed in addition to, or alternately from, nozzles 593. Thelimiting system may employ pressure holding limiters, such as burstplugs or erosion disks 595, to ensure that sufficient pressure isretained to continue to move the plug through the string and to move thesleeves even after one or more caps are sheared. After the cap 546 a isremoved, the port is opened except as restricted by disk 595 (FIG. 8E)and the port will not fully open (FIG. 8F) until the disk breaks down.In the illustrated embodiment, disk 595 includes a fluid escape port 592that initially allows a flow of high pressure fluid F1 to escape throughthe disk. Port 592 creates a site for erosion and the erosion breaksdown the disk over time, until it is fully opened. When fully opened, afull orifice frac flow F2, can be injected through port 516 and nozzle593 therein (FIG. 8F). The eroding properties of the disk may beselected to ensure that port 516 remains substantially closed for longenough that the plug has moved through its intended path and opened allintended kobe plugs.

The sleeve can include an end 580 c formed to engage against a stopshoulder 581 a in the tubing string wall. End 580 c or wall 581 a may beselected, as by angular forming, to properly direct the sleeve radiallyoutwardly to prevent inward collapsing damage to the sleeve.

If sleeve 580 or any part of the sleeve is not suitably recessed in thewall 515 of the kobe sub, the protruding part can be milled out, asdesired (FIG. 8G). The sleeve, recess 581 and/or shoulder 581 a may beselected to keep the sleeve from turning during milling. For example, asshown, end 580 c may be formed, as by sharpening, tipping with spikes orcutters, faceting, etc., to become rotationally locked in the string tokeep it from turning.

A ball seat can be employed below the lower most kobe plug in the seriesthat creates a seal with plug 542 to isolate the series of opened kobeplugs so that fluid can be diverted to the opened ports.

While the embodiment of FIG. 8 shows a system using collapsible ballseats on the sleeves, it is to be appreciated that similar result couldbe achieved by employing a collapsible ball with appropriatenon-collapsing ball seats on the kobe shielding sleeves. The collapsibleball may be selected to squeeze through the ball seats but in so doingexert a sufficient force to move the sleeves. Where a plurality ofsleeves is to be opened in one operation, the collapsible ball may beformed to be resilient and therefore able to act on a plurality of seatsalong the string.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are know or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 USC 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

1. A kobe sub comprising: a tubular body connectable into a wellboretubing string, the tubular body including a wall including an outersurface and an inner surface defining an inner bore and a port throughthe wall; a kobe plug installed in the port with a cap portionaccessible in the inner bore, a base mounted in the port and connectedto the cap portion, a channel extending through the base and closed bythe cap portion; and a shielding structure in the inner bore to shieldthe cap portion from protruding into the inner bore.
 2. The kobe sub ofclaim 1 wherein the shielding structure includes a shielding wallextending alongside the cap portion.
 3. The kobe sub of claim 2 whereinthe shielding wall is a sidewall of a slot formed in the wall innersurface.
 4. The kobe sub of claim 3 wherein the sidewall defines a slotdepth and the slot depth is greater than the height of the cap portionsuch that the cap portion is fully recessed in the slot.
 5. The kobe subof claim 3 wherein the slot includes a return overlying a portion of thecap portion.
 6. The kobe sub of claim 2 wherein the shielding wallincludes an extension of the wall inner surface overlying a portion ofthe cap portion.
 7. The kobe sub of claim 1 wherein the shieldingstructure is a sleeve installed in the inner bore and positioned tooverlie at least a portion of the cap portion.
 8. The kobe sub of claim7 wherein the sleeve is moveable to abut against the cap portion toremove the cap portion from a sealing position on the base.
 9. The kobesub of claim 7 wherein the sleeve includes a shoulder against which aport opening tool pushes to move the sleeve.
 10. The kobe sub of claim 9wherein the shoulder is a ball seat and the port opening tool is a plugsized to land and seal in the ball seat.
 11. The kobe sub of claim 1further comprising a cap portion capturing mechanism to capture the capportion from becoming loose in the inner bore after the cap portion isremoved from the base.
 12. The kobe sub of claim 11 wherein the capportion capturing mechanism pushes the cap portion outwardly through thechannel toward the outer surface.
 13. The kobe sub of claim 11 whereinthe cap portion capturing mechanism stores the cap portion in the sub.14. The kobe sub of claim 1 wherein the shielding structure is a sleeveinstalled in the inner bore and positioned to overlie at least a portionof the cap portion and the cap portion capturing mechanism is a cavityin the sleeve that retains the cap portion after the cap portion isremoved from the base.
 15. The kobe sub of claim 1 wherein the shieldingstructure is a slot formed in the wall inner surface and the cap portioncapturing mechanism includes a return overlying a portion of the capportion to prevent the cap portion from passing out of the slot after itis removed from the base.
 16. The kobe sub of claim 1 further comprisinga flow limiting device in the channel for at least restricting flowthrough the channel and for providing a delay before the port becomesfully open to fluid flow.
 17. The kobe sub of claim 15 wherein the flowlimiting device is removed by erosion.
 18. A method for forming a fluidchannel through a tubing string wall, the method comprising: installinga tubing string in a wellbore, the tubing string including a tubularwall including an outer surface and an inner surface defining an openinner bore and a port through the wall; a kobe plug installed in theport with a cap portion accessible in the inner bore, a base mounted inthe port and connected to the cap portion, a channel extending throughthe base and closed by the cap portion; and a shielding structure in theinner bore to shield the cap portion from protruding into the innerbore; introducing a port opening tool into the inner bore; manipulatingthe tool to overcome the shielding structure; and removing the capportion to open the channel and form the fluid channel though the tubingstring wall.
 19. The method of claim 18 wherein the shielding structureincludes a shielding wall extending alongside the cap portion andmanipulating the tool includes reaching past the shielding wall toremove the cap portion.
 20. The method of claim 19 wherein the shieldingwall is a sidewall of a slot formed in the inner surface andmanipulating the tool includes reaching into the slot to remove the capportion.
 21. The method of claim 18 wherein the shielding structure is asleeve installed in the inner bore and positioned to overlie at least aportion of the cap portion and manipulating the tool includes moving thesleeve.
 22. The method of claim 21 wherein moving the sleeve causes thesleeve to abut against the cap portion and shear the cap portion fromthe base.
 23. The method of claim 18 wherein manipulating the toolincludes manipulating a string on which the tool is carried.
 24. Themethod of claim 18 wherein manipulating the tool includes landing thetool against the shielding structure and applying a pressure drivenforce to overcome the shielding structure.
 25. The method of claim 18further comprising capturing the cap portion such that the cap portionis prevented from becoming loose in the inner bore.
 26. The method ofclaim 25 wherein capturing the cap portion includes collecting the capportion in the tool.
 27. The method of claim 25 wherein capturingincludes storing the cap portion in the tubing string.
 28. The method ofclaim 25 wherein capturing includes pushing the cap portion out throughthe channel away from the inner bore.
 29. The method of claim 18 furthercomprising manipulating the tool to open a second kobe plug axiallyspaced from the kobe plug to form a second fluid channel through thetubing string wall.
 30. The method of claim 29 wherein the tool movesfrom the kobe plug to the second kobe plug while moving along the tubingstring.
 31. The method of claim 29 further comprising delaying a fullopening of the channel until after the tool has opened both the kobeplug and the second kobe plug.
 32. A tubing string system forinstallation in a wellbore comprising: a tubular body including atubular wall with an outer surface and an inner surface defining aninner bore and a port through the tubular wall; a kobe plug installed inthe port with a cap portion accessible in the inner bore, a base mountedin the port and connected to the cap portion, a channel extendingthrough the base and closed by the cap portion; a shielding structure inthe inner bore to shield the cap portion from protruding into the innerbore; and a port opening tool for overcoming the shielding structure andremoving the cap portion from the base.
 33. The tubing string system ofclaim 32 wherein the shielding structure includes a shielding wallextending alongside the cap portion and the port opening tool includes astructure for reaching past the shielding wall to remove the capportion.
 34. The tubing string system of claim 33 wherein the shieldingwall is a sidewall of a slot formed in the wall inner surface and theport opening tool includes a structure for reaching into the slot toremove the cap portion.
 35. The tubing string system of claim 34 whereinthe sidewall defines a slot depth and the slot depth is greater than theheight of the cap portion such that the cap portion is fully recessed inthe slot.
 36. The tubing string system of claim 34 wherein the slotincludes a return overlying a portion of the cap portion and the portopening tool includes a structure for reaching past the return and intothe slot to remove the cap portion.
 37. The tubing string system ofclaim 33 wherein the shielding wall includes an extension of the wallinner surface overlying a portion of the cap portion and the portopening tool includes a structure for reaching past the extension toremove the cap portion.
 38. The tubing string system of claim 32 whereinthe shielding structure is a sleeve installed in the inner bore andpositioned to overlie at least a portion of the cap portion and the portopening tool includes a structure for moving the sleeve to remove thecap portion.
 39. The tubing string system of claim 32 wherein the portopening tool is carried on a string.
 40. The tubing string system ofclaim 32 wherein the port opening tool is conveyed by fluid pressure.41. The tubing string system of claim 32 wherein the port opening toolis a plug and the sleeve includes a seat for retaining the plug suchthat a pressure driven force is generated to move the sleeve.
 42. Thetubing string system of claim 32 further comprising a flow limitingdevice in the channel for at least restricting flow through the channeland for providing a delay before the port becomes fully open to fluidflow.
 43. The tubing string system of claim 42 wherein the flow limitingdevice is removed by erosion.
 44. The tubing string system of claim 32further comprising a cap portion capturing mechanism to capture the capportion from becoming loose in the inner bore after the cap portion isremoved from the base.
 45. The tubing string system of claim 44 whereinthe cap portion capturing mechanism pushes the cap portion outwardlythrough the channel toward the outer surface.
 46. The tubing stringsystem of claim 44 wherein the cap portion capturing mechanism storesthe cap portion in the sub.
 47. The tubing string system of claim 44wherein the cap portion capturing mechanism collects the cap portion inthe port opening tool.
 48. The tubing string system of claim 32 furthercomprising a second kobe plug axially spaced along the tubing stringfrom the kobe sub and a second shielding structure for the second kobeplug; and the port opening tool also is operable to overcome the secondshielding structure to open the kobe plug.
 49. The tubing string systemof claim 32 wherein the kobe plug is one in a series of kobe plugsspaced axially along the tubing string, the series of kobe plugs beingconfigured to be acted upon by the port opening tool in sequence in oneoperation. 50-119. (canceled)